This invention relates to an improved latch useful in apparatus for heat-sealing plastic film, and is disclosed herein, by way of example, as applied to L-sealers as shown in U.S. Pat. No. 3,490,981. The invention disclosed therein consisted of an L-sealer with a latching system for holding a sealing arm down in sealing position, the latching system being shown in detail in FIGS. 4, 5 and 6 of the aforesaid patent.
As shown in the aforesaid patent, an L-shaped sealing arm is provided for forming an L-shaped seal. The arm is pivoted at the free ends of the L-shaped sealing element for movement about a horizontal axis into engagement with a matching L-shaped lower sealing bed.
Product is wrapped in a web of thermoplastic film that has a fold at the rear side so that there is a continuous web under the bottom of the product around the rear and over the top. A transverse seal is made at the beginning of the package with the L-shaped sealing element, then the product is put into the film and moved into the sealing station where the two remaining sides are sealed simultaneously by the L-shaped sealing element. From then on, each L-shaped seal will produce a complete package having seals on three sides with the fold at the rear of the web on the fourth side.
L-sealers prior to the aforesaid U.S. Pat. No. 3,490,981 were either operated by hand or by pneumatic means or by hydraulic means. In the case of the pneumatic and hydraulic sealers, the cylinders were usually mounted to the rear of the machine, although some cylinders were mounted overhead. The cylinder and piston that produced the force had to have sufficient power to produce a uniform clamping pressure on the sealing arm in order for a good seal to be made in plastic film. Since the cylinder and piston had to perform both the function of moving the sealing arm closed and providing clamping pressure, the cylinder and piston had to be of relatively large diameter and long stroke and, consequently, required a large volume of air to operate. In many plants, it is difficult to find this volume of compressed air and, in any case, it is expensive to compress air.
The apparatus in the aforesaid patent was designed to use a relatively small diameter cylinder and, hence, a relatively small volume of compressed air to move the sealing arm from the inoperative to the operative position. Since the force exerted by the cylinder was small, a pneumatically actuated cam-type latch was used which would grip the sealing arm when the upper sealing jaws were brought into engagement with lower sealing beds and pull it down the remaining fraction of an inch so that the sealing arm would apply the appropriate pressure to the seal bed, and, therefore, make a good seal on the film.
In lieu of this pneumatic cam-latch, competitors used electromagnets. In each case, the electromagnet was mounted to a bracket that was fastened to the frame of the sealer and the armature was fastened to the sealing arm. The sealing arm would be brought down and pressed onto the bed by hand or would be brought down by a relatively small diameter pneumatic cylinder and, when the sealing arm reached the bottom of a stroke, the electromagnets would grip the armature. When the cycle was completed, the current was turned off, the electromagnet de-energized, and the sealing arm was again raised.
The pneumatic cam-latch had several disadvantages. The film could get caught between the cam and roller and could become torn. The timing of the cam was relatively critical because the sealing arm tended to bounce if it closed too hard and, when the sealing arm bounced up, the latch missed the engagement roller and the sealing arm missed a cycle. In order for a pneumatic cam-latch sealer to operate properly, the sealing arm had to be adjusted to operate smoothly and the height above the operative position that the pneumatic cam-latch is triggered had to be adjusted to make the latch engage properly. Since there was considerable hammering on contact of the latch with the roller, there had always been considerable wear on the latch parts.
The electromagnetic holding device required adjustment of the magnet height or armature height and had to be about correct to obtain the proper clamping force. Since the seal bed of an L-sealer frequently has to be replaced and, since the seal bed was comprised of rubber and sponge rubber composition that vary in thickness, the electromagnetic latching device would not always apply proper pressure. The greatest disadvantage of the electromagnetic latching system was that, although it could hold the sealing arm down in place once it was forced into place by some other force, it did not have the force to pull the sealing arm closed and to provide sufficient sealing pressure for making optimum seals. In other words, the effective distance of the magnetic field force was so small that just a few thousandths of an inch of distance between the armature and the electromagnet would cause the magnetic latch not to pull down properly.
To sum up, the pneumatic cam-latch produced the proper sealing pressure, but with certain mechanical problems and the electromagnetic latch could not be depended upon to reliably produce the proper sealing pressure.